Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

555 timer circuit issue

  1. Jun 9, 2012 #1

    I have designed a circuit based on one that I found on the internet (attached as example.gif). This circuit worked well when I built it on a breadboard.

    I made a few minor modifications to the example circuit (attached as schem1.bmp) and I am having a few issues that I do not understand. The modifcations are simple, as all I have done really is add additional LEDs to the circuit in both the LED1 and LED2 positons of the example and also add an additional circuit on the same board that shares the same ground.

    I have the following issues with the circuit that I have built:

    1) The timer function no longer works at all. I have tried swapping resistors after the 555 pin3, as well as the componnets in R1, R2, and C1.

    2) The brightnesses between the LED1 and LED2 positions are inconsistent, when a test voltage is applied.

    3) The additional circuit on the board is a simple resistor/led circuit. The issue is that when the LEDs are turned on, you can visually see effects from the frequency of the 555 timer.

    Each positive lead was tested using the digital output of an Arduino microcontroller at 5v.

    Any help would be greatly appreciated, as it has me stumped...

    Attached Files:

  2. jcsd
  3. Jun 9, 2012 #2
    Powered at 5V, you may be only able to source/sink 20-50mA (check the datasheet).
    This will light one or two standard LEDs. If you really want to light a lot of LEDs using output of 555 you can try:
    1 - powering 555 from higher VCC (some 555's can go up to 16V).
    2 - using low power LEDs
    3 - buffering output of 555
  4. Jun 16, 2012 #3
    Could you elaborate on how I would 'buffer' the output of such a circuit? I am a novice electronics hobbyist who is self-taught, and I have probably not taught myself all of the essentials so my apologies if these questions sound rudimentary or even ludicrous.
  5. Jun 16, 2012 #4
    No apologies needed.

    Buffering means adding extra parts that can handle higher current to drive more LEDs. There's a lot of ways to do this. The most common way is to add transistors so that the 555 switches the transistors instead of the LEDs and the transistors then turn the LEDs in and off.

    Google "transistor buffer 555" or "push pull buffer for logic level LED drive" and you should get lots of hits.
  6. Jun 16, 2012 #5

    Thank you for the REALLY fast reply. That is much appreciated.

    I also have a quick question about resolving issue #3 in my post. I have been doing some reading, and I think the appropriate solution would be to use a diode of some type. (The main issue here is that these are two lighting circuits sharing a same ground, and you can visually see the frequency pulse in the second circuit of the shared ground.) Would you know if that would resolve my issue? I believe that the diodes forward direction would be from circuit 2 to the shared ground to eliminate any signal traveling from the ground to circuit 2. Does any of this sound correct in any way?

    Thanks so much!
  7. Jun 16, 2012 #6
    Diode will not help that. The problem is that your power source has a non-negligable resistance. When the 555 LEDs turn on, the voltage at the other LEDs droops a little and they get dimmer.

    What you need a is a regulator like a 7805 ahead of the whole circuit. Make sure you heatsink it properly.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook